1. Field of the Invention
The present invention generally relates to cable-line communications systems, and more particularly to an on-hook transmission circuit provided in a cable line, such as a digital line.
2. Description of the Prior Art
Generally, in a cable telephone line, a called subscriber picks up the handset in response to a call, and information can be transmitted in the off-hook state. Recently, an advanced system has been proposed in which information can be transferred in the on-hook state without picking up the handset in response to a call. This is called an on-hook transmission, and information can be transferred without current flowing in a subscriber terminal when a call is received. Such an on-hook transmission is effective for a case where the called subscriber wishes to know who is calling before picking up the handset.
However, the two wires of the telephone line connected to the telephone terminal are kept open-circuited in the on-hook state, and therefore a modification is needed to achieve the on-hook transmission services.
FIG. 1 is a circuit diagram of part of a conventional subscriber circuit (also called line circuit) connected to a telephone terminal (not shown). A first wire 1 and a second wire 2 form a subscriber telephone line. A first bias circuit J1 applies a bias voltage to the first wire 1, and a second bias circuit J2 applies a bias voltage to the second wire 2. The first bias circuit J1 has a first terminal 3a, a second terminal 3b and a third terminal 3c. The second bias circuit J2 has a first terminal 4a, a second terminal 4b and a third terminal 4c.
The collector of an npn transistor Q1 and the first terminal 3a of the first bias circuit J1 are connected to the first wire 1. The emitter of the npn transistor Q1 is connected to a voltage source VBB via a resistor RA1. The second terminal 3b of the first bias circuit J1 is connected to the voltage source VBB, and the third terminal 3c thereof is connected to the non-inverting input terminal of an operational amplifier A1. The inverting input terminal of the operational amplifier A1 is connected to a node at which the emitter of the npn transistor Q1 and the resistor RA1 are connected in series to each other. The output terminal of the operational amplifier A1 is connected to the base of the npn transistor Q1.
The collector of a pnp transistor Q2 and the first terminal 4a of the second bias circuit J2 are connected to the second wire 2. The emitter of the pnp transistor Q2 is connected to ground GND via a resistor RB1. The second terminal 4b of the second bias circuit J2 is connected to the ground GND. The third terminal 4c of the second bias circuit J2 is connected to the non-inverting input terminal of an operational amplifier A2. The inverting input terminal of the operational amplifier A2 is connected to a node at which the emitter of the pnp transistor Q2 and the resistor RB1 are connected in series to each other. The output terminal of the operational amplifier A2 is connected to the base of the pnp transistor Q2.
It will now be assumed that the bias voltage output via the third terminal 3c of the first bias circuit J1 is VA1, and the bias voltage output via the third terminal 4c of the second bias circuit J2 is VB1. In an operating mode, the bias voltage VB1 appears at the pnp transistor Q2 because of the imaginary or virtual short-circuit of the operational amplifier A2. Similarly, the bias voltage VA1 appears at the pnp transistor Q1 because of the virtual short-circuit of the operational amplifier A1. The difference between the ground potential and the bias voltage VB1 and the difference between the power source voltage VBB and the bias voltage VA1 are equal to each other.
Hence, a feed current IL flowing in the subscriber terminal connected across the first line 1 and the second line 2 is equal to (VB1-G)/RB1, where G is the ground potential, and RB1 is the resistance value of the resistor RB1. When the first wire 1 and the second wire 2 are open-circuited, the feed current does not flow in the subscriber terminal. Hence, the first wire 1 is maintained at VBB, and the second wire 2 is maintained at G. In this case, a signal cannot be transferred because an AC signal (information) cannot be transferred in the state where the feed current does not flow in the subscriber terminal. For the above-mentioned reason, the circuit shown in FIG. 1 is not capable of providing the on-hook transmission services.
FIG. 2 shows a modification of the circuit shown in FIG. 1 in order to enable the on-hook transmission services. In FIG. 2, parts that are the same as parts shown in FIG. 1 are given the same reference numbers or symbols. The emitter of a pnp transistor Q3 is connected to the first wire 1 via a resistor RA2. The first terminal 3a of the first bias circuit J1 is connected to the first wire 1. The collector of the pnp transistor Q3 is connected to the voltage source VBB.
The second terminal 3b of the first bias circuit J1 is connected to the voltage source VBB. The third terminal 3c of the first bias circuit J1 is connected to the non-inverting input terminal of an operational amplifier A3. The inverting input terminal of the operational amplifier A3 is connected to the emitter of the pnp transistor Q3. The emitter of an npn transistor Q4 is connected, via a resistor RB2, to the second wire 2 and the first terminal 4a of the second bias circuit J2. The collector of the npn transistor Q4 is connected to the ground GND. The second terminal 4b of the second bias circuit 4b is connected to the ground GND, and the third terminal 4c thereof is connected to the non-inverting input terminal of an operational amplifier A4. The inverting input terminal of the operational amplifier A4 is connected to the emitter of the npn transistor Q4. The output terminal of the operational amplifier A4 is connected to the base of the npn transistor Q4.
It will now be assumed that the bias voltage output via the third terminal 3c of the first bias circuit J1 is VA2, and the bias voltage output via the third terminal 4c of the second bias circuit J2 is VB2. In an operating state, the bias voltage VB2 appears at the npn transistor Q4 because of the virtual short-circuit of the operational amplifier A4. The bias voltage VA2 appears at the pnp transistor Q3 because of the virtual short-circuit of the operational amplifier A3. The difference between the bias voltage VB2 and the potential of the second wire 2 is equal to the difference between the bias voltage VA2 and the potential of the first wire 1. Hence, the subscriber terminal connected across the first wire 1 and the second wire 2 is maintained in the loop state, the feed current IL passing through the subscriber terminal is equal to (the difference between the bias voltage VB2 and the potential of the second wire 2)/RB2. In the above case, when the bias voltages VB2 and VA2 are respectively a few volts, a voltage (G-VB2) appears at the second wire 2, and a voltage (VA2-VBB) appears at the first wire 1. Hence, it is possible to provide the on-hook transmission services by means of the circuit configuration shown in FIG. 2.
However, a problem occurs when the circuit configuration shown in FIG. 2 is connected to a subscriber terminal which operates using the difference between the voltage of the first wire 1 and the second wire 2 obtained in the open-circuit state. There is a type of subscriber terminal that supervises the difference between the first and second wires 1 and 2 (normally, the above difference is 48 volts). The voltage VBB, (VA2+VB2) corresponding to the difference between the first and second wires 1 and 2 is fed to such a subscriber in the open-circuit state. Hence, this voltage applied to the subscriber terminal is out of the specification defined for it, and the subscriber terminal cannot operate or malfunctions.